This invention relates to a control system which controls the locking and unlocking of a differential gear mechanism.
It is well known to provide vehicles, such as farm tractors, with differentials which can be selectively locked or unlocked. In a simple form, differential lock control systems are in use wherein a solenoid-operated differential lock control valve is controlled by a single, manually-operated floor switch. However, with such a system, the vehicle operator must continuously depress the floor switch in order to keep the differential locked. On both row-crop and four-wheel-drive (4WD) tractors with differential locks, a locked differential can interfere with the steering of the tractor. One solution to this problem, applicable to row-crop tractors, has been to unlock the differential in response to a left or right wheel brake application since the brakes are often used to assist the turning of a row-crop tractor. Such a system is described in U.S. Pat. No. 2,874,790 (Hennessey).
Another solution to this problem has been to control the locking of the differential as a function of the relative angle between mechanical components of the tractor steering system. An example of this is also shown in the Hennessey patent. Now, it is often necessary to operate a farm tractor in a sidehill situation wherein the tractor front end is constantly angled slightly uphill to compensate for the tendency of the tractor to drift in a downhill direction. In such a situation, when little steering effort is required to counteract the downhill drift, it would be desirable to have the differential remain locked. However, a steering angle dependent system, such as shown in the Hennessey patent, may automatically unlock the differential in this sidehill situation, regardless of whether such unlocking is desirable or necessary, if the steering angle exceeds the particular angle beyond which the differential is unlocked.
A solution to this latter problem in copending U.S. patent application Ser. No. 503,970, filed June 13, 1983 and assigned to the assignee of the present invention. However, the system described therein requires more than one relay and requires that an electrical circuit be completed through the components of a steering valve to obtain steering-responsive differential unlocking. Although operable, this is not an optimum solution because hydraulic fluid is an insulator and an open circuit electrical failure condition in the system could prevent a desired unlocking of the differential.